There exist a number of analog depth sounders which provide a display of depth in terms of the angular position of a lighted neon bulb with respect to a zero, usually at the top of the display. These depth sounders in general incorporate a neon bulb mounted on a rotary disk in which the neon bulb is actuated by the return of sonar signals which have been projected into the water and which are reflected either by the bottom of the ocean or by fish. The disk is driven by a synchronous motor which is timed in accordance with the transmission of the signals from the transducer used with the depth sounder such that the angular position of the actuated neon bulb corresponds to the depth immediately underneath the vessel utilizing the depth sounder. A circumferential scale is conventionally provided on the display to relate angle to sensed depth. As is conventional, the depth sounder is provided with an intensity control and the operator reduces the intensity of the neon bulb to the point at which only two lines are visable, namely the zero line and the first depth line. It will be appreciated that multiple lines can be viewed on the depth sounder display when multiple bounces of the sonar signal occur. By adjustment of the intensity level, it is possible to adjust the depth sounder such that only one unambiguous indication of depth is displayed.
While these depth sounders are indeed a great aid to navigation, especially in narrow channels, it is oftentimes inconvenient to glance at the display while trying to navigate in a crowded channel. It will be appreciated that glancing at the depth sounder to readout the depth is not only inconvenient, but also can result in accidents due to glancing away from the scene.
It is therefore desirable that an audible indication of depth be provided so that the operator of the vessel need not divert his attention from the situation at hand.
Moreover, since depth sounders are frequently used in navigation, it is likewise desirable to provide an audible depth indication for the vessel's navigator whose attention is drawn away from the chart he is using when he must look up at the depth sounder in order to ascertain the depth of the water over which the vessel is currently passing.